1. Field of the Invention
The present invention relates to a semiconductor device that has a circuit including a thin film transistor (hereinafter, referred to as a TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display panel is mounted as a component.
It is to be noted that the term “semiconductor device” in the present specification indicates a general device capable of functioning by utilization of semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic devices are all included in the semiconductor device.
2. Description of the Related Art
In recent years, a technique has attracted attention, in which a thin film transistor (TFT) is formed using a semiconductor thin film (a thickness of about several nm to several hundred nm) that is formed over a substrate having an insulating surface. Such a thin film transistor is widely applied to electronic devices such as ICs or electro-optical devices, and development thereof as a switching element of an image display device is particularly urgent.
A liquid crystal display device is known as an image display device. Active matrix-type liquid crystal display devices have been commonly used because a high-definition image can be obtained by the case of using the active matrix-type device as compared with the case of using a passive-type liquid crystal display device. In the active matrix-type liquid crystal display device, when pixel electrodes arranged in matrix are driven, a display pattern is formed on a screen. As for details, when a voltage is applied between a selected pixel electrode and an opposite electrode that corresponds to the selected pixel electrode, optical modulation of a liquid crystal layer arranged between the pixel electrode and the opposite electrode is performed, and this optical modulation is recognized as a display pattern by observers.
In a general transmission-type liquid crystal display device, a liquid crystal layer is arranged between a pair of substrates (a first substrate and a second substrate), a first polarizing element is arranged on an outer surface side, which is not adjacent to the liquid crystal layer, of the first substrate (substrate provided with a pixel electrode), and a second polarizing element is arranged on an outer surface side, which is not adjacent to the liquid crystal layer, of the second substrate (opposite substrate).
When a color filter is used for displaying full color, the color filter is generally arranged on a difference surface from the surface of the substrate (opposite substrate) where the polarizing element is arranged. In other words, the color filter is generally arranged between the opposite substrate and the liquid crystal layer.
A technique for forming a color filter successively with the use of rollers is disclosed in Reference 1 (Japanese Published Patent Application No. H8-234018). Moreover, in Reference 1, at least two or more films are attached to a front surface or a rear surface of a film is disclosed, the at least two or more films being selected from an optical thin film having a color filter function, an optical thin film having a fluorescent function, an optical thin film having an antireflection function, an optical thin film having a phase compensation function, an optical thin film having a light shielding function, a transparent thin film having conductivity, or a transparent thin film having an adhesive function.
In recent years, high-definition of a display image and improvement in display quality in liquid crystal display devices have been demanded. When the observer sees display of the liquid crystal display device in a vertical direction to a substrate surface of the display device, display quality in accordance with design of the liquid crystal display device can be obtained since arrangement of pixel electrodes and position of color filters (one colored layer with respect to one pixel) approximately correspond to each other. However, when the observer sees display in a diagonal direction to the substrate surface, apparent misalignment occurs due to parallax. In addition, since light leakage is caused due to this parallax, contrast of display is lowered. Accordingly, a viewing angle characteristic is caused, in which display quality is degraded when the observer sees display in a diagonal direction to the substrate surface. In particular, if a pixel size is miniaturized as compared with a thickness of the substrate in order to achieve high-definition of the display image, the parallax becomes remarkable, and a viewing angle for ensuring an allowable display characteristic is narrowly limited.
Further, easiness for seeing display of the liquid crystal display device depends on brightness of surrounding light. In a case of using a mobile-type liquid crystal display device or an outdoor-type liquid crystal display device, surrounding light is changed from brightness of sunlight in daytime to brightness in nighttime. Thus, such a liquid crystal display device is formed as a transmission-type or a semitransmission-type liquid crystal display device having a backlight to uniform easiness for seeing display, and then the convenience becomes favorable. However, when an intense light source is used as a backlight to uniform easiness for seeing the liquid display device under external light in daytime, light leakage due to parallax becomes increased; therefore, it is difficult to hold contrast of display.